With a recent version of Sympa (>=5 ?), wws manages aliases automatically in /etc/mail/sympa_aliases (or /etc/mail/sympa.aliases in Debian).
However, for instance in Debian, when using it with exim4, you’ll need to let Exim know it should use /etc/mail/sympa_aliases.

It seems to me that the following snippet should do the trick. Just copy the whole block of the ”system_aliases” driver just bellow it, naming the new one ”sympa_aliases”, for instance, and replace the ”lsearch{/etc/aliases}” by ”lsearch{/etc/mail/sympa.aliases}” in the new driver. You may also need to add a user = sympa definition too in that block.

As I understand it, if the first ”system_aliases” redirect driver fails, it will do the same with the next one (”sympa_aliases”).

In Debian, depending on the choices of configuration for exim4, you may need to add a new file (split config), or change /etc/exim4/exim4.conf.template (single file), and then update the setup with the update-exim4.conf command (man 8 update-exim4.conf for more details). More details may be found in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=169102.

You may also need to add sympa to the list of users in MAIN_TRUSTED_USERS variable, maybe (to be confirmed).

AND in your virtual domains tool manager (ie : @domain2.tld), you just have to create a forwarder like this :
mylist → mylistdomain2tld

It’s very simple and it work…. I suppose that your Exim is already well configured to serve messages to your virtual domains … an easy-to-use tool for managing vdomains: http://www.ex-parrot.com/~chris/vmail-sql/